Soil Calculator Guide UK 2025

Planning a garden project in the UK? Whether you're filling raised beds, laying a new lawn, creating flower borders, or top-dressing your vegetable patch, calculating the right amount of soil is essential. Ordering too little means project delays and additional delivery costs, while ordering too much wastes money and leaves you with piles of excess soil to dispose of.

This comprehensive guide explains how to calculate soil quantities accurately, helping you choose between bags and bulk delivery, and ensuring you get the right type of soil for your specific project.

Understanding Soil Volume Calculations

Calculating soil volume is straightforward once you understand the basic formula. The key is measuring accurately and allowing for settlement.

Step-by-Step Calculation

1. Measure your area: Use a tape measure to get length and width in metres
2. Determine depth: Decide how deep you need soil (see recommendations below)
3. Calculate volume: Multiply length × width × depth
4. Add settlement allowance: Add 10-15% for natural compaction
5. Convert to tonnes or bags: Multiply by 1.5 for tonnes, or by 40 for 25L bags

Common Raised Bed Calculations

Raised beds are increasingly popular in UK gardens for vegetables, herbs, and flowers. Here are pre-calculated volumes for common sizes:

Recommended Soil Depths

Different garden applications require different depths of topsoil. Using too little can result in poor plant growth, while excessive depth wastes money.

Buying Options: Bags vs Bulk

The most economical way to buy soil depends on the quantity you need. Here's a cost comparison for UK gardeners:

Types of Soil and Compost

Choosing the right type of soil or growing medium is as important as calculating the quantity:

• Screened topsoil: General purpose, good for borders, lawns, and filling beds. Should be BS3882 certified.
• Turf dressing: Finely screened sandy loam, specifically for lawn preparation and top-dressing.
• Vegetable compost: Rich in organic matter and nutrients, ideal for edible gardens.
• Multi-purpose compost: Lighter and more porous, best for containers and pots.
• Peat-free compost: Environmentally friendly alternative, increasingly popular and recommended.
• Bark mulch: For weed suppression and decorative purposes, not for planting.

Accounting for Settlement

Fresh soil always settles over time as air pockets compress and organic matter breaks down. This is particularly important for raised beds.

UK Soil Suppliers

Popular UK suppliers for topsoil and compost include:

• National: Rolawn, British Sugar TOPSOIL, Compost Direct
• Builders' merchants: Jewson, Travis Perkins, Buildbase
• Garden centres: B&Q, Wickes, Homebase, local independent centres
• Aggregates suppliers: Local quarries often supply screened topsoil

How Soil Volume Calculations Work

Calculating soil requirements is fundamentally a volume calculation: you multiply the area to be covered by the desired depth. For rectangular beds, this is straightforward — length x width x depth. For circular areas, you use pi x radius squared x depth. The result gives you a volume in cubic metres, which is the standard unit for ordering bulk soil in the UK.

However, raw volume alone does not account for several practical factors. Freshly delivered soil will settle by approximately 10-20% over the first few months as it compacts under its own weight and through watering. For raised beds, you should add a 15% settlement allowance to your calculated volume. For areas that will be turfed, a 10% allowance is usually sufficient since the turf itself provides some compression resistance.

Weight conversion is also essential for ordering and delivery planning. One cubic metre of topsoil typically weighs between 1.2 and 1.7 tonnes, depending on moisture content and composition. Sandy soils sit at the lighter end, while clay-rich soils are heavier. Most UK suppliers sell topsoil by the tonne or in bulk bags (typically 0.75-0.85m3 per bag, weighing roughly 850kg-1 tonne). Knowing both your volume and approximate weight requirements helps you compare prices accurately between suppliers.

UK Soil Types and Regional Considerations

The UK has remarkably diverse soil types across relatively short distances, which significantly affects gardening success. The British Geological Survey identifies six main soil types across England, Wales, and Scotland: clay, sandy, silty, peaty, chalky, and loamy. Your region's underlying geology determines your garden's natural soil character. South-east England tends towards chalky and clay soils, the Midlands features heavy clay, East Anglia has sandy and silty soils, and the West Country offers more loamy conditions.

When ordering topsoil to improve your garden, you should consider your existing soil type. Adding pure topsoil to heavy clay creates a layering effect where water pools at the boundary between soil types. The Royal Horticultural Society (RHS) recommends incorporating organic matter such as well-rotted compost or manure to improve clay soils rather than simply topping with new soil. For sandy soils that drain too quickly, adding soil with higher organic content helps retain moisture and nutrients.

For raised beds and container gardening — increasingly popular in UK urban gardens — a blended growing medium often outperforms pure topsoil. A common UK recipe is 60% screened topsoil, 30% composted green waste, and 10% sharp sand. This blend provides good drainage, nutrient retention, and root penetration. Many UK suppliers now offer pre-blended raised bed soil, though mixing your own is usually more cost-effective for larger projects.

Frequently Asked Questions About Soil Calculations

How Deep Should Topsoil Be for a New Lawn?

For a new lawn from seed, you need a minimum of 100mm (10cm) of quality topsoil, though 150mm (15cm) is recommended for the best results. For turf laying, 100mm is generally sufficient as the turf brings its own root zone. If you are laying turf over existing soil that is compacted or of poor quality, remove or rotavate the top 50mm first, then add 150mm of fresh topsoil. For a typical UK back garden of 50 square metres, this means ordering approximately 5-7.5 cubic metres of topsoil, equivalent to roughly 6-10 tonnes.

What Is the Difference Between Topsoil and Compost?

Topsoil is the upper layer of natural earth, screened and sometimes blended to remove stones and debris. It provides the bulk structure for planting areas and contains minerals and some organic matter. Compost, by contrast, is decomposed organic material — either garden waste, food waste, or a combination. Compost is far richer in nutrients but breaks down over time and should not be used alone as a growing medium for most plants. In practice, you need topsoil for volume and structure, and compost as an amendment to boost fertility. A ratio of 70-80% topsoil to 20-30% compost works well for most UK garden applications.

Can I Reuse Soil from Other Parts of My Garden?

Yes, reusing existing garden soil is both economical and environmentally sound. When excavating for patios, paths, or foundations, set aside the topsoil (the darker upper layer, usually 150-300mm deep) separately from the subsoil. Topsoil can be stockpiled for up to 12 months if kept in heaps no taller than 1.5 metres — taller heaps compress the lower layers, killing beneficial organisms. Screen reused soil through a 10-20mm mesh to remove stones, roots, and debris. Be cautious with soil from areas where persistent weeds like Japanese knotweed, horsetail, or bindweed have been present, as these can regenerate from small root fragments.

James Mitchell, ACCA

Chartered Accountant & Former HMRC Advisor

James is a Chartered Certified Accountant (ACCA) specialising in UK personal taxation and financial planning. With over 12 years in practice and a background as a former HMRC compliance officer, he brings authoritative insight to complex tax topics.